High-strength steel sheet excellent in deep drawing characteristics and method for production thereof

Abstract

A high-strength steel sheet is useful for applications to automobile steel sheets and the like and has excellent deep drawability, a tensile strength (TS) of as high as 440 MPa or more, and a high r value (average r value ≧1.2), and a process for producing the steel sheet. The steel sheet has a composition containing, by % by mass, 0.010 to 0.050% of C, 1.0% or less of Si, 1.0 to 3.0% of Mn, 0.005 to 0.1% of P, 0.01% or less of S, 0.005 to 0.5% of Al, 0.01% or less of N, and 0.01 to 0.3% of Nb, the Nb and C contents in steel satisfying the relation, (Nb/93)/(C/12)=0.2 to 0.7, and the balance substantially including Fe and inevitable impurities. The steel microstructure contains a ferrite phase and a martensite phase at area ratios of 50% or more and 1% or more, respectively, and the average r value is 1.2 or more.

1. A high-strength steel sheet having excellent deep drawability, an average r value of 1.2 or more, and a composition, which is free of V, comprising by % by mass:
 C: about 0.010 to about 0.050%; 
 Si: about 1.0% or less; 
 Mn: about 1.0 to about 3.0%; 
 P: about 0.005 to about 0.1%; 
 S: about 0.01% or less; 
 Al: about 0.005 to about 0.5%; 
 N: about 0.01% or less; 
 Nb: about 0.01 to about 0.3%; and 
 
       the balance substantially including Fe and inevitable impurities, the Nb and C contents in steel satisfying the relation, (Nb/93)/(C/12)=0.2 to less than 0.5 (wherein Nb and C represent the contents (% by mass) of the respective elements) and C in solution is, 47 to 83% of total C content and the steel microstructure containing a ferrite phase and a martensite phase at area ratios of about 50% or more and about 1% or more, respectively, and having a grain size of 8 μm or less. 
     
     
       2. The high-strength steel sheet having excellent deep drawability according to  claim 1 , wherein the steel sheet satisfies the following relation between normalized X-ray integrated intensity ratios of (222) plane, (200) plane, (110) plane, and (310) plane parallel to the sheet plane at a ¼ thickness of the steel sheet:
 P(222)/{P(200)+P(110)+P(310)} 1.5 (wherein P(222), P(200), P(110), and P(310) are the normalized X-ray integrated intensity ratios of the (222) plane, (200) plane, (110) plane, and (310) plane, respectively, parallel to the sheet plane at a ¼ thickness of the steel sheet). 
 
     
     
       3. The high-strength steel sheet having excellent deep drawability according to  claim 1 , further comprising at least one of Mo, Cr, Cu, and Ni in a total of about 0.5% by mass or less in addition to the composition. 
     
     
       4. The high-strength steel sheet having excellent deep drawability according to  claim 1 , further comprising about 0.1% by mass or less of Ti in addition to the composition, the contents of Ti, S, and N satisfying the following relation:
   (Ti/48)/{(S/32)+(N/14)}≦2.0 
 
       (wherein Ti, S, and N represents the contents (% by mass) of the respective elements). 
     
     
       5. The high-strength steel sheet having excellent deep drawability according to  claim 1 , further comprising a plated layer on a surface thereof. 
     
     
       6. A process for producing a high-strength steel sheet having excellent deep drawability, the process comprising a hot rolling step of finish-rolling a steel slab by hot rolling at a finisher delivery temperature of about 800° C. or more to form a hot-rolled sheet having a grain size of 8 μm or less and coiling the hot-rolled sheet at a coiling temperature of about 400 to about 720° C., a cold rolling step of cold-rolling the hot-rolled sheet to form a cold-rolled sheet, and a cold-rolled sheet annealing step of annealing the cold-rolled sheet at an annealing temperature of about 800 to about 950° C. and then cooling the annealed sheet in a temperature range from the annealing temperature to about 500° C., at an average cooling rate of about 5° C./s or more, the steel slab having a composition, which is free of V, comprising by % by mass:
 C: about 0.010 to about 0.050%; 
 Si: about 1.0% or less; 
 Mn: about 1.0 to about 3.0%; 
 P: about 0.005 to about 0.1%; 
 S: about 0.001% or less; 
 Al: about 0.005 to about 0.5%; 
 N: about 0.01% or less; and 
 Nb: about 0.01 to about 0.3%; 
 
       the Nb and C contents in steel satisfying the relation, (Nb/93)/(C/12)=0.2 to less than 0.5 (wherein Nb and C represent the contents (% by mass) of the respective elements) and C in solution is 47 to 83% of total C content. 
     
     
       7. A process for producing a high-strength steel sheet having excellent deep drawability, the process comprising a hot rolling step of hot-rolling a steel slab to form a hot-rolled sheet having an average crystal grain size of 8 μm or less, a cold rolling step of cold-rolling the hot-rolled sheet to form a cold-rolled sheet, and a cold-rolled sheet annealing step of annealing the cold-rolled sheet at an annealing temperature of about 800 to about 950° C. and then cooling the annealed sheet in a temperature range from the annealing temperature to about 500° C. at an average cooling rate of about 5° C./s or more, the steel slab having a composition, which is free of V, comprising by % by mass:
 C: about 0.010 to about 0.050%; 
 Si: about 1.0% or less; 
 Mn: about 1.0 to about 3.0%; 
 P: about 0.005 to about 0.1%; 
 S: about 0.01% or less; 
 Al: about 0.005 to about 0.5%; 
 N: about 0.01% or less; and 
 Nb: about 0.01 to about 0.3%; 
 
       the Nb and C contents in steel satisfying the relation, (Nb/93)/(C/12)=0.2 to less than 0.5 (wherein Nb and C represent the contents (% by mass) of the respective elements) and C in solution is 47 to 83% of total C content. 
     
     
       8. The process for producing the high-strength steel sheet having excellent deep drawability according to  claim 6  or  7 , wherein the steel slab further contains at least one of Mo, Cr, Cu, and Ni at a total of about 0.5% by mass or less in addition to the composition. 
     
     
       9. The process for producing the high-strength steel sheet having excellent deep drawability according to  claim 6 , wherein the steel slab further contains about 0.1% by mass or less of Ti in addition to the composition, the contents of Ti, S, and N satisfying the following relation:
   (Ti/48)/{(S/32)+(N/14)}≦2.0 
 
       (wherein Ti, S, and N represents the contents (% by mass) of the respective elements). 
     
     
       10. The process for producing the high-strength steel sheet having excellent deep drawability according to  claim 6 , further comprising a plating step of forming a plated layer on a surface of the steel sheet after the cold-rolled sheet annealing step. 
     
     
       11. The high-strength steel sheet having excellent deep drawability according to  claim 2 , further comprising at least one of Mo, Cr, Cu, and Ni in a total of about 0.5% by mass or less in addition to the composition. 
     
     
       12. The high-strength steel sheet having excellent deep drawability according to  claim 2 , further comprising about 0.1% by mass or less of Ti in addition to the composition, the contents of Ti, S, and N satisfying the following relation:
   (Ti/48)/{(S/32)+(N/14)}≦2.0 
 
       (wherein Ti, S, and N represents the contents (% by mass) of the respective elements). 
     
     
       13. The high-strength steel sheet having excellent deep drawability according to  claim 3 , further comprising about 0.1% by mass or less of Ti in addition to the composition, the contents of Ti, S, and N satisfying the following relation:
   (Ti/48)/{(S/32)+(N/14)}≦2.0 
 
       (wherein Ti, S, and N represents the contents (% by mass) of the respective elements). 
     
     
       14. The high-strength steel sheet having excellent deep drawability according to  claim 2 , further comprising a plated layer on a surface thereof. 
     
     
       15. The high-strength steel sheet having excellent deep drawability according to  claim 3 , further comprising a plated layer on a surface thereof. 
     
     
       16. The high-strength steel sheet having excellent deep drawability according to  claim 4 , further comprising a plated layer on a surface thereof. 
     
     
       17. The process for producing the high-strength steel sheet having excellent deep drawability according to  claim 7 , further comprising a plating step of forming a plated layer on a surface of the steel sheet after the cold-rolled sheet annealing step. 
     
     
       18. The process for producing the high-strength steel sheet having excellent deep drawability according to  claim 8 , further comprising a plating step of forming a plated layer on a surface of the steel sheet after the cold-rolled sheet annealing step. 
     
     
       19. The process for producing the high-strength steel sheet having excellent deep drawability according to  claim 9 , further comprising a plating step of forming a plated layer on a surface of the steel sheet after the cold-rolled sheet annealing step.